Add a README

Author: jmillikin

README.md

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+# Bazel buuld rules for GNU Bison
+
+## Overview
+
+```python
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+http_archive(
+    name = "rules_m4",
+    urls = ["https://github.com/jmillikin/rules_m4/releases/download/v0.1/rules_m4-v0.1.tar.xz"],
+    sha256 = "7bb12b8a5a96037ff3d36993a9bb5436c097e8d1287a573d5958b9d054c0a4f7",
+)
+load("@rules_m4//m4:m4.bzl", "m4_register_toolchains")
+m4_register_toolchains()
+
+http_archive(
+    name = "rules_bison",
+    # See https://github.com/jmillikin/rules_bison/releases for copy-pastable
+    # URLs and checksums.
+)
+load("@rules_bison//bison:bison.bzl", "bison_register_toolchains")
+bison_register_toolchains()
+```
+
+```python
+load("@rules_bison//bison:bison.bzl", "bison_cc_library")
+bison_cc_library(
+    name = "hello",
+    src = "hello.y",
+)
+cc_binary(
+    name = "hello_bin",
+    deps = [":hello"],
+)
+```
+
+```python
+load("@rules_bison//bison:bison.bzl", "bison_java_library")
+bison_java_library(
+    name = "HelloJavaParser",
+    src = "hello_java.y",
+)
+java_binary(
+    name = "HelloJava",
+    srcs = ["HelloJava.java"],
+    main_class = "HelloJava",
+    deps = [":HelloJavaParser"],
+)
+```
+
+## Other Rules
+
+```python
+load("@rules_bison//bison:bison.bzl", "bison")
+bison(
+    name = "hello_bin_srcs",
+    src = "hello.y",
+)
+cc_binary(
+    name = "hello_bin",
+    srcs = [":hello_bin_srcs"],
+)
+```
+
+```python
+genrule(
+    name = "hello_gen",
+    srcs = ["hello.y"],
+    outs = ["hello_gen.c"],
+    cmd = "M4=$(M4) $(BISON) --output=$@ $<",
+    toolchains = [
+        "@rules_bison//bison:toolchain",
+        "@rules_m4//m4:toolchain",
+    ],
+)
+```
+
+## Toolchains
+
+```python
+load("@rules_flex//bison:bison.bzl", "bison_common")
+load("@rules_m4//m4:m4.bzl", "m4_common")
+
+def _my_rule(ctx):
+    bison_toolchain = bison_common.bison_toolchain(ctx)
+    m4_toolchain = m4_common.m4_toolchain(ctx)
+    ctx.actions.run(
+        executable = bison_toolchain.bison_executable,
+        inputs = depset(transitive = [
+            bison_toolchain.files,
+            m4_toolchain.files,
+        ]),
+        env = {"M4": m4_toolchain.m4_executable.path},
+        # ...
+    )
+
+my_rule = rule(
+    _my_rule,
+    toolchains = [
+        bison_common.TOOLCHAIN_TYPE,
+        m4_common.TOOLCHAIN_TYPE,
+    ],
+)
+```